For very small cracks in metal, the key is usually contrast rather than just camera resolution. A standard inspection approach is dye penetrant testing: apply a penetrant that seeps into fractures, then inspect under suitable lighting. Ultraviolet penetrant dyes can make tiny defects much easier to see because they increase contrast.

For imaging, keep the camera facing straight down and use adjustable angled lighting so surface defects cast contrast clearly. You can also improve visibility with optical filters on the camera lens, which can be more effective than trying to recover contrast later in software.

So the practical setup is: a camera that can tether to a computer, controllable low-angle lighting, and—if your process allows it—penetrant dye plus appropriate illumination/filtering. The exact camera matters less than having good lighting geometry and enough contrast to make the fracture obvious in the captured image.